The present invention relates generally to frequency measuring systems and, more particularly, to a method for developing a signal representative of the instantaneous frequency of an alternating current (AC) signal.
There are a number of frequency measuring systems available in today's art. In one form, the cycles of the signal being measured are counted for some period of time to give a direct value of the frequency. At low frequencies, this method is undesirable because long count periods are needed to give adequate resolution. A somewhat more sophisticated approach is to measure the period of the waveform and invert the value of the period to get a frequency indication. Although this method provides better results than the direct counting method described above, it does not give the optimum result, particularly when frequency is low or when it is rapidly decreasing in value.
Typically, in the prior art it is necessary to await the completion of at least one-half cycle of the alternating current signal being measured in order to get an indication of the frequency. As such, so long as the frequency is high, there is at least a fairly accurate indication of frequency at the end of the half cycle. When the frequency is decreasing in value and the new period is longer than that previously being measured, it is apparent that an increase in time is required for the frequency indication.
In U.S. Pat. No. 4,454,470, "Method and Apparatus for Frequency Measurement of an Alternating Current Signal" by F. H. Boettner et al., issued June 12, 1984, which patent and the teachings thereof are specifically incorporated hereinto by reference, there is described a scheme which develops the values alternately representing, respectively, the periods of successive half cycles of the alternating current signal. By comparing two such values, the last completed value with a value presently being generated, and by providing an output signal representing the value representing the longer period, there is provided a signal which is more representative of the instantaneous frequency when the frequency of the alternating current signal is decreasing. This is because that, although the frequency value is not exactly known, as soon as the value representing the presently being measured half cycle exceeds the value of the period of the half cycle last measured, there is provided an indication that the period is getting longer and hence that the frequency is decreasing.
Even the system of U.S. Pat. No. 4,454,470, however, can be considerably in error since it is always necessary to wait until the end of a half cycle to get an actual measure of the frequency. That is, during the period of a half cycle, there is no indication of the instantaneous frequency. It is further realized that in these prior art systems which require the completion of a half cycle for development of a frequency signal, the value provided is one which is proportional to the average frequency over the half cycle and not the frequency at the end of a half cycle. This is most readily visualized with reference to a system in which a counter develops a count proportional to the length of a half period. It is readily apparent that reliance upon the total count developed during a full half cycle gives a result representative of the average frequency during the half cycle and not the frequency at the end of the half cycle.